1. Field of the Invention
This invention relates generally to stereotactic frames for use in stereotactic surgery, and is more particularly concerned with a frame holder for positioning a stereotactic frame with respect to an imaging system.
2. Description of the Prior Art
Stereotactic surgery requires that a particular point, for example in the head, be located by coordinates in three dimensional space. Currently, this is accomplished through the use of a stereotactic frame which has a coordinate system in conjunction therewith. With the patient in the stereotactic frame, images are made of the patient, using computer aided tomography (CAT), magnetic resonance imaging (MRI) or some comparable imaging system. The reference marks specifying the coordinate system of the stereotactic frame are visible on the images permitting calculation of coordinates of a chosen point.
Those skilled in the art will understand that the stereotactic frame defines a volume of three dimensional space, thereby allowing a positional description of any point in that defined volume to be made via the specified coordinate system. As a result, the surgeon must rely on the coordinate system to compute the precise location intended for surgery. It will therefore be understood that the attitude of the stereotactic frame with respect to the imaging device is important. A misalignment between the stereotactic frame and the imaging device will cause the coordinate reference markers on the stereotactic frame to be misaligned in the images. The mathematics of calculating the coordinates of a given point in the space defined by the stereotactic frame are significantly more complex without proper alignment of the stereotactic frame and the imaging system.
In making images of a patient's head using MRI, the head is placed within a "head coil" which is a transceiver for the radio-frequency (RF) signals of interest. The head coil defines a central opening for receiving the patient's head, and for receiving the head with a stereotactic frame thereon for stereotactic surgery. It is here that alignment is important. The stereotactic frame is preferably precisely aligned with the head coil in order to facilitate coordinate calculation.
The usual efforts at aligning a stereotactic frame with the head coil comprise attempting to prop up various portions of the frame with towels or the like. It will be understood that the material used must be carefully selected. Metals will redirect, or shield, the signals of interest, and numerous other materials will produce signals of their own. Ferrous metals cannot safely be placed in the room holding the MRI scanner due to the strength of the corresponding magnetic field and the risk of resulting movement. Thus, towels are commonly used in attempts to hold the frame in place. Since towels are deformable, they must be packed carefully. Even so, the frame frequently moves with respect to the head coil after imaging has begun. A shift in the frame may render calculation more difficult if the shift is before the actual scan, or may require a new scan if the shift is during the scan.